The present invention relates to an antifouling system for preventing marine organisms from attaching to surfaces of a structure exposed to seawater and, more particularly, to an antifouling system capable of generating oxygen by the agency of an electrical catalyst provided on a surface of a structure exposed to seawater in order to prevent marine organisms from attaching to the surface of the structure exposed to seawater.
In some power plant that uses seawater as cooling water, blue mussels, barnacles, hydrozoans or marine plants, which will be inclusively referred to as xe2x80x9cmarine organismsxe2x80x9d, tend to attach to an inlet and an outlet pipe plates for heat-exchanger-holding heat transfer pipes. These marine organisms block up end parts of the heat transfer pipes so that passage of a cleaning sponge through the heat transfer pipes is obstructed and/or that insides of the heat transfer pipes are narrowed. Therefore, the power plant is frequently forced to stop its operation unavoidably to remove the marine organisms. These marine organisms are liable to attach to pipe plates and/or heat transfer pipes that are made of titanium and that have corrosion-resistance in seawater, more than to pipe plates and heat transfer pipes made of a copper alloy.
In a steel water chamber lined with a rubber lining, larvae of marine organisms passed through a strainer net may attach to the rubber liner, and the larvae of marine organisms repeats growing on the rubber lining and falling off the rubber lining. This may clog heat transfer pipes for cooling.
Measures taken to exterminate the marine organisms and to prevent attachment of the marine organisms to the structure (hereinafter referred to as xe2x80x9cantifouling measuresxe2x80x9d) include pouring chlorine or chlorine compound into the ambient region of the sea around the structure, coating the structure with antifouling paint containing toxic-ion-producing pigment and producing of toxic ions such as chlorine ions or copper ions by electrolysis of seawater.
Although these antifouling measures exercise effective antifouling functions, amount and concentration is not easy to be managed, and the concentration is liable to be excessively increased in anticipation of reliable antifouling effect. Consequently, it is highly possible that the antifouling measures cause environmental pollution. Therefore, it is recent trend that use of such antifouling measures is inhibited or controlled.
Recently, many research workers and engineers are engaged in development of antifouling measures that do not use toxic substances. For example, antifouling silicone paints are nonpolluting and nontoxic but have antifouling effect. However, antifouling silicone paints have not been prevalently used because their drawbacks including shortening of service life of antifouling silicone paints by contact with foreign matters such as shells, high costs of work for application of the antifouling silicone paints, difficulty in finding suitable applying means capable of simply and easily applying the antifouling silicone paints to structures having a large surface area and/or existing structures, and reduction of the antifouling effect of antifouling silicone paints when flow of seawater is stopped.
A method mentioned in JP-B No. Hei 01-46595 forms an electrical catalyst film, mainly consisting of a mixed crystal of metals of platinum group or a mixture of oxides of those metals, on the surface of a titanium heat exchanger or the like to be exposed to seawater, and generates a sufficient amount of oxygen substantially without generating chlorine gas by electrolysis using the titanium heat exchanger as an anode to control attachment of marine organisms to the heat exchanger or formation of scales on the heat exchanger.
However, since this prior art method forms the electrical catalyst film over the surfaces of titanium structural members to be exposed to water or seawater and uses the titanium structural members as an anode, other metallic members, such as a water chamber or pipes that are usually made of steels and lined with rubber linings, of the heat exchanger electrically connected to the titanium structural members are similarly loaded as the anode. If, by any chance, the rubber linings or the like should be damaged, a current flows through a part of the metallic members corresponding to a damaged part of the rubber linings so that a structural member of a metal other than titanium may be subject to abnormal corrosion.
Moreover, this prior art method conducts an electrical resistance heating process at temperatures in the range of 350 to 450xc2x0 C. for several hours for activation of the electrical catalyst. This electrical resistance heating process is possible to damage the structure by generated heat and/or thermal stresses, and requires an enormous cost. Accordingly, this prior art method has not been prevalently practiced.
As mentioned above, the prior art technique mentioned in JP-B No. Hei 01-46595 coats the titanium members of a heat exchanger directly with the electrical catalyst film, heats the titanium members at temperatures in the range of 350 to 450xc2x0 C. for several hours by means of an electrical resistance heating or the like for the thermal activation, and uses the same as an anode. Therefore, it is possible that the structure is damaged by heat applied thereto and/or thermal stresses induced therein. In addition, such electrical resistance heating requires an enormous cost.
Generally, in the titanium heat exchangers, only the heat transfer pipes and the pipe plates are titanium members, but the body, the water chamber, suction pipes for carrying seawater to the heat exchanger and discharge pipes for discharging used seawater into the sea are made of steels. Since the steel water chamber, the steel suction pipes and the steel discharge pipes are electrically connected to the titanium members, the same are subject to galvanic corrosion when exposed to seawater and may be heavily corroded. Therefore, the surfaces of the steel members that may be wetted with seawater are coated with rubber linings for preventing corrosion.
If, by any chance, a rubber lining of a steel member is damaged, the titanium member electrically connected to the steel member must be cathodically loaded by a cathodic protection method that lowers potential of the steel member to a protection potential thereof. However, since the aforesaid prior art technique uses the titanium member as an anode, the steel water chamber, the suction pipes and the discharge pipes connected therewith are anodically loaded, and hence the cathodic protection method cannot be applied in principle so that a current flows through a part of the steel member corresponding to a damaged part of the rubber lining and that the steel member is abnormally corroded.
It is an object of the present invention to provide an antifouling system capable of easily forming an electrical catalyst on a surface of a titanium pipe plate or the like of a heat exchanger, without applying heat to the titanium pipe plate or the like by electric resistance heating or the like, capable of electrically isolating the electrical catalyst from structural members such as the titanium pipe plates, and capable of preventing abnormal corrosion of a part of a metal member corresponding to a damaged part of a rubber lining or the like coating the metal member, by employing a cathodic protection method even when the rubber lining or the like is damaged by some rare accident.
According to the present invention, an antifouling system that generates oxygen on a seawater-exposed surface of a structure to be exposed to seawater to prevent attachment of marine organisms to the seawater-exposed surface of the structure comprises: an anode forming member bonded to the seawater-exposed surface of the structure to be exposed to seawater via an insulating adhesive; an electrochemically active and stable electrical catalyst coated on the anode forming member; a conductive member disposed so as to be wetted with seawater; and an external DC power supply having a positive terminal connected to the anode forming member or the electrical catalyst, and a negative terminal connected to the conductive member, and including an automatic potential controller; wherein a potential difference between the positive and the negative terminals of the external DC power supply is set in order to generate oxygen in seawater while suppressing generation of chlorine in seawater.
According to the present invention, the anode forming member coated in advance by the electrical catalyst can be easily bonded to the seawater-exposed surface of the structure with the insulating adhesive at an ordinary temperature. Thus, the structure may not be damaged by thermal stresses or the like. In addition, because of the insulating adhesive, the anode forming member can be electrically insulated from the structure such as the titanium pipe plates. Thus, abnormal corrosion of a part of a metal member corresponding to a damaged part of a rubber lining or the like protecting the metal member electrically connected to the titanium pipe plate or the like can be prevented even when the rubber lining is broken by some rare accident.
Preferably, an insulating sheet is disposed between the seawater-exposed surface of the structure to be exposed to seawater and the anode forming member.
Preferably, the electrical catalyst is a single substance of a metal of the platinum group, a metal oxide of the platinum group, cobalt oxide or manganese oxide, a mixed crystal substance thereof or a complex substance thereof.
Preferably, the anode forming member is a titanium member, more preferably, a titanium sheet. Preferably, the titanium sheet has a thickness in the range of 0.1 to 0.3 mm and can be wound in a coil. Preferably, the titanium sheet is divided into a plurality of pieces, and conductive tapes are used to electrically connect adjacent pieces of the titanium sheet.
Preferably, the insulating adhesive is an elastic adhesive mainly containing a denatured and an epoxy resin and providing a stable bond strength when the temperature of seawater is in the range of 0 to 50xc2x0 C.
When the structure to be exposed to seawater is a titanium heat exchanger having a plurality of titanium heat transfer pipes and one or more titanium pipe plates supporting the plurality of titanium heat transfer pipes, it is preferable that the anode forming member is provided with a plurality of openings corresponding to diameters of the plurality of titanium heat transfer pipes.
According to the present invention, an antifouling system that generates oxygen on a seawater-exposed insulating part of a structure to be exposed to seawater to prevent attachment of marine organisms to the seawater-exposed insulated part of the structure comprises: an anode forming member provided at the seawater-exposed insulating part of the structure; an electrochemically active and stable electrical catalyst coated on the anode forming member; a conductive member disposed so as to be wetted with seawater; and an external DC power supply having a positive terminal connected to the anode forming member or the electrical catalyst, and a negative terminal connected to the conductive member, and including an automatic potential controller; wherein the potential difference between the positive and the negative terminals of the external DC power supply is set in order to generate oxygen in seawater while suppressing generation of chlorine in seawater.
According to the present invention, since the oxygen can be generated around the seawater-exposed insulating part of the structure while suppressing the generation of chlorine, the attachment of marine organisms to the structure can be prevented.
In this antifouling system also, it is preferable that the electrical catalyst is a single substance of a metal of the platinum group, a metal oxide of the platinum group, cobalt oxide or manganese oxide, a mixed crystal substance thereof or a complex substance thereof.
Similarly, it is preferable that the anode forming member is a titanium member, more preferably, a titanium sheet. Preferably, the titanium sheet has a thickness in the range of 0.1 to 0.3 mm and can be wound in a coil. Preferably, the titanium sheet is divided into a plurality of pieces, and conductive tapes are used to electrically connect adjacent pieces of the titanium sheet.
The seawater-exposed surface of the insulating part of the structure to be exposed to seawater may be a wall surface coated with a rubber or resin lining.
When the structure to be exposed to seawater is a concrete structure, it is preferable that the conductive member is a reinforcing bar for the concrete structure.